Various changes to README and release files.

 * CHANGES.txt: Renamed from NEWS, added 0.6.1 section, fixed 0.6.0.
 * MANIFEST.in: Remove ChangeLog, rename NEWS to CHANGES.txt.
 * README.rst: Renamed from README.rst, added badges for Travis-CI and
   PyPI.
 * setup.py: Reflect README rename, add 3.3 to classifiers.